A manufacturing process of semiconductor devices may involve an etching process using a plasma, wherein the processing condition for the etching process, such as selection of the processing gas used, is determined based on the types of the film to be etched and the under film. For example, in order to etch a SiO2 film (silicon oxide film) or a SiCOH film (hydrogenated silicon oxycarbide film), which contains silicon and oxygen to serve as an insulating film, there has been known a method for converting a gas containing C (carbon) and F (fluorine) into a plasma to thereby allow SiO2 or SiCOH to be removed by a reaction with a CF based or a CHF based radical. For example, there has been known a method for performing an etching by using a CxFy gas (fluorocarbon gas), CO gas (carbon monoxide gas), O2 gas (oxygen gas) and a nonreactive gas in order to form a contact hole at a SiO2 film (silicon oxide film) serving as an insulating film (see Japanese Patent Laid-open Application No. H6-338479, paragraphs 0026, 0039, 0050 and FIG. 1: Reference 1). Though N2 gas serving as a dilution gas may be employed as the nonreactive gas, Ar gas is also widely employed because the Ar gas tends to be readily converted into a stable flow of plasma, and to facilitate dissociation of the CF based gas to thereby increase CF based active species.
Meanwhile, with a recent trend for high integration of semiconductor devices, various problems have occurred. For instance, patterns are becoming finer and a distance between neighboring holes or grooves, which are recesses to be etched, is getting narrower. Further, it is expected that the distance between, for example, holes for burying capacitance devices would be reduced to 200 nm or less in a future design rule. In case the holes are closely located to such extent, however, a new problem may occur.
Referring to FIGS. 13 to 15, the new problem will be described. In FIG. 13, reference numerals 11, 12 and 13 represent a SiO2 film, a resist mask (resist film) and holes, respectively. In case of performing an etching on a substrate having such surface layers by using a processing gas including C4F8 gas and a nonreactive gas, the inner peripheral surfaces of the holes 13 may become undulated because resist portions between the holes 13 are very thin. As a result, a desired shape of the holes 13, e.g., a circular shape or an elliptical shape, cannot be obtained. FIG. 14 shows poor shapes of circular holes 13 formed at the resist mask, which are viewed from the top after the etching. If the holes 13 of the resist mask 12 are undulated as shown therein, their shapes are transferred to holes of the SiO2 film 11, resulting in an occurrence of striation, i.e., a formation of grooves elongated in a depth direction at the inner peripheral surfaces of the corresponding holes. Given that the holes themselves are minute, various problems may be caused if the shapes of the holes become poor and irregular. For instance, a desired capacitance cannot be obtained in case the holes are for use in burying capacitance devices, and an expected conductivity cannot be obtained in case the holes are formed as contact holes, resulting in a reduction of yields in both cases.
Further, if it is difficult to avoid cutting of angled portions of the resist mask 12 by an etching gas, and if resist portions between the holes 13 are very thin, the angled portions 14 may be cut at both sides, as shown in FIG. 15, and interferences therebetween may cause poor shapes of holes 13 which may be transferred to the holes of the SiO2 film, thereby reducing the yields. Moreover, there is also a likelihood that the selectivity, i.e., the ratio of an etching rate of the SiO2 film to an etching rate of the resist mask 12 may be reduced. As patterns become finer, the wavelength of an exposure light source becomes shorter, so that it is required to further reduce the thickness of the resist mask. For this reason, it is undesirable to have the selectivity reduced. This problem also occurs for, e.g., a SiCOH film, which is receiving considerable attention as an interlayer insulating film with a low dielectric constant, as well as the SiO2 film. In case of the SiCOH film, there may occur another problem that upper portions of recesses are enlarged in case patterns become finer. If such a problem occurs in case the recesses are, e.g., via holes (through holes) in which materials for a conductive channel for connecting an upper layer circuit and a lower layer circuit are to be buried, the irregular diameters of the holes may cause capacitances between the via holes when they are crowded, resulting in a delay in a signal transfer. The reason for this is still being investigated but it is conjectured that the Ar gas used in the plasma etching causes widening of the resist mask's openings which in turn causes this problem.
Moreover, below the SiCOH film, there is an under film (etching stop layer) also referred to as a stop layer for stopping the etching of the SiCOH film. For example, a silicon carbide film, a silicon nitride film, a SiON film or a SiCO film is used as the under film. In case of etching the SiCOH film, there occurs a problem that the selectivity of the SiCOH film against the under film tends to be low. Thus, a so-called overetching of the under film may be performed at a part of a wafer surface which is etched to reach a bottom of the SiCOH film first, and an etching of the under film is conducted because the selectivity is low, so that a lower layer wiring is etched through the under film. As a result, the in-surface uniformity in the thickness of the under film is reduced. In such event, resistances at connection portions for connecting the lower layer circuit and the upper layer circuit become different across the wafer surface, resulting in a decrease of yields. The above-mentioned problem is not considered in Reference 1.
Further, Reference 2 (i.e., Japanese Patent Laid-open Application No. H11-168090, paragraphs 0013, 0014, 0019 and 0020) discloses that, in case of etching a BPSG (boron phosphorous silicate glass) film after forming a pattern of holes with a diameter of 0.18 μm on a mask, xenon (Xe) gas is added to C4F8 gas used as an etching gas in order to prevent a damage to an underlying silicon film. However, Reference 2 does not consider the problems occurring when the interval between the holes is narrowed, the selectivity problem and the problem occurring in case of etching a SiCOH film.